Corrosion is a very important problem in the Municipal Solid Waste Incinerator (MSWI) superheaters. This problem causes the plant stops and tube replacements, promoting the loss of energy generation rate. The main corrosion agent is the chlorine deposits. HVOF coatings have been sprayed to improve corrosion resistance of the superheaters inside the MSWI boilers. Inconel 625 and Hastelloy C22 alloys have been sprayed as a powder feedstock material. The spray process has been analyzed by a Spray Watch system that allows carrying out the temperature and velocity measurements of the particles in flight in order to optimize the spraying process. The produced coatings have been characterized by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) to evaluate the porosity and oxidation produced in the spray process. Laboratory oxidation test have revealed the formation of Cr 2 O 3 , NiO and Fe 2 O 3 as corrosion products as well as Microanalysis by Energy dispersion Spectroscopy (EDS) composition profiles have been used to evaluate the penetration of the corrosion front inside the coating. The Nickel alloys show a good resistance in aggressive environment test and can be a good solution for the corrosion problems in the superheater tubes in MSWI.

Producing nanostructured materials through metastable phases is an interesting novel route in the field of ceramic materials. Due to their small grain size and uniform structure, these nanostructured bulk materials exhibit very interesting properties. Metastable coatings can be produced starting from microstructured powders through atmospheric plasma spray (APS) technique, followed by a quenching route. The initial powders are melted during the spraying and deposited over a substrate that is quenched with liquid nitrogen (LIN) feeders, producing metastable coatings. The thermal sprayed coatings have been characterized using XRD, SEM, FESEM and EDS in the Thermal Spray Centre (CPT) of the University of Barcelona. The properties of such coatings have been also studied obtaining promising results.

Steel tubes oxidation causes an important problem in Municipal Solid Waste Incinerator plants (MSWI). This work shows a possible solution for this problem through High Velocity Oxygen Fuel (HVOF) thermal spray coatings. A comparison between powder and wire thermal spray coatings (with the same composition) has been done. These optimised coatings have been compared through their microstructure, wear properties (ASTM G99-90, ASTM G65-91), and erosion-corrosion (E-C) resistance. Results of the different EC tests designed in the Thermal Spray Centre, have been evaluated by X Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Disperse Spectroscopy (EDS). Commercial bulk material with similar composition of Ni Based coatings has also been tested. The protection mechanisms of these materials have been assessed after studying the results obtained by HVOF coatings and bulk material. Ni based HVOF coatings are a promising alternative to the MSWI conventional protection against chlorine environments.

Recent research in the field of nanostructured ceramic materials has underscored the importance of using feedstock powders with metastable phases. During material consolidation the metastable structure evolves into a dual structure where the grain growth stops when the crystallites reach their respective equilibrium grain size. The mutual suppression of the grain growth is a direct consequence of the immiscibility of the two phases in the solid state. Due to their small grain size and uniform structure, the nanocomposites exhibiting very interesting properties, are formed. Metastable structures can be produced using atmospheric plasma spray (APS) technique. The main target of this work has been the synthesis of Al 2 O 3 -TiO 2 metastable powder through atmospheric plasma spraying (APS) and quenching route. The metastable phases have been produced from micron-sized feedstock powders and the effect of different raw powders have also been evaluated. The metastable phases have been characterised by XRD, SEM and FESEM analysis. The availability of producing nanostructured materials through these metastable phases have been studied. The phase and morphology evolution from this metastable phases to nanostructures has been observed.

The results for joints obtained by dynamic diffusion bonding of a 90MnCrV8 high strength steel coated with WC-Co are shown in the present work. This high strength steel substrate was coated with WC-Co, sprayed by HVOF technique (Diamond Jet Hybrid DJH-2700) using propylene as fuel gas at different conditions. The dynamic diffusion bonding was carried out in a high frequency furnace, all joints were made in air. Before doing the joints, the steel was coated with Ni and Cu by electrochemical processes in order to obtain a soft 20 m interlayer of Ni30Cu alloy. Microstructure and reacted zones in the joints were investigated by means of Scanning Electron Microscopy (SEM) and Dispersive X Ray Spectrometry (EDX). In all joints different reacted zones can be distinguished, caused by diffusion processes which take place during the joint tests. The mechanical properties of the joints were quantified in a tensile machine, using a constant load of 0.1 MPa·s -1 . All joints broke by the WC-Co coating zone by delamination processes. The fracture surface was studied by SEM-EDX in order to know the fracture mechanism of the joints. The maximum tensile strength obtained confirm a very promising technology for industrial applications.

Bulk, welded or laser cladded Stellite based materials are widely used in chemical industry due to their excellent combined properties against corrosion and wear processes and as a superalloy they have a high melting point and are designed to withstand high temperature for long periods of time. Problems related with poor inter-splat bonding arise when this kind of materials are sprayed with HVOF because it is characterized by having relatively low flame temperatures and high particle speed, and so dwell time may not be long enough to melt or soften Stellite particles completely. For these reasons, typical structured composed by a superposition of non-melted particles are obtained when spraying with propylene fuel gas. The aim of this work is to study the effect of using Hydrogen as a fuel gas and also to study the effect of the variation of the propylene flame characteristics. The coatings are characterized by SEM-EDS, XRD, and ASTM G99-90 sliding wear test has been done to compare the coatings. SWLI has been used to measure the volume lost after the wear test. Splat morphology studies have been also done in order to compare the melting behavior of the impinging particles.

The use of HVOF to produce surface coatings is much known. However, the spraying of inner parts or using it to reproduce a mold form is not always easy due to the high bonding strength of HVOF coatings. On the other hand, plasma formed parts are being used in the aerospatial industry for many years partly because the ease of the releasing process. Using the experience and the method developed in the CPT some years ago, some new materials have been sprayed to obtain thick axy-symmetrical self-standing forms. The process involves mechanical removal method and for these reason, the mould can be used several times with minimal maintenance. Amongst the releasing methods, the more simple is mechanical but it could turn out to be complex if the adhesion by impingement takes place too strongly. Hence, the impingement of the material is studied in this work. The materials are mechanical blends of WC-Co with Co-based alloys. However, prior to spraying, the most suitable material is chosen for a concrete purpose. The work includes a study of the spraying parameters in the structure and the properties of the coatings. XRD, SEM-EDS, roughness, microhardness, ASTM G 99-90 and ASTM G 65-91 wear test are performed to characterize the samples.

Over the last years, the grain size reduction to the nanometric scale has experienced an increasing interest because of its enhanced properties. This study is based on WC-Co cermet materials, which have a great application in the wear resistance materials field. In the Thermal Spray Center (CPT) of the University of Barcelona, two coatings using nanostructured and bimodal WC-Co cermet powders have been obtained by means of High Velocity Oxygen-Fuel (HVOF) technique. Both were sprayed under the same spraying parameters. Structural characterization for both feedstock materials and coatings have been performed using XRD, SEM and TEM. The friction wear resistance of the coatings has been studied by Ball-on-Disk test (ASTM G99-90), and the abrasive wear resistance has been quantified by Rubber-Wheel test (ASTM G65-91). Finally, the corrosion resistance has been studied by electrochemical techniques and a salt fog spray test. The nanostructured coating shows more hardness, but the bimodal coating shows better abrasive and friction wear resistance. Both coatings shows good corrosion resistance, better than the conventional coatings of WC-Co. The bimodal powder have the added advantage of being not as expensive as the nanostructured powder, and even providing better properties.

Thermal and kinetic history of particles inside the jet plume of atmospheric plasma spray (APS) provide the particles with a certain velocity and temperature prior to the impact onto the substrate. Adjusting these velocity and temperature allows to control final coating's properties. The properties of the obtained coatings mostly depend on particle's conditions prior to the impingement. A set of experiments were done in order to optimize an atmospheric plasma process by means of an online monitoring system. The study is done by changing some spraying parameters that certainly affect the velocity and the temperature of the in-flight particles. Plasma gas flow rates and spraying distance were the parameters changed in this work, whereas all the other spraying conditions were kept constant. On-line monitoring system Spray-Watch (Oseir Ltd.) was used to achieve the parameters optimization by the measurement of velocity, temperature and spatial distribution of the Ni20Cr inflight particles. The study of the coated samples includes microstructure characterization by means of Optical Microscopy (OM), Scanning Electron Microscopy (SEMEDS) and X-ray diffraction to study the effect of the spraying parameters in phase composition. Roughness measurements and mechanical properties are also included.

The development of new spraying processes has increased the demand for high quality protecive coatings. Many thermal spraying processes have been developed to obtain coatings for a wide spectrum of materials and substrates. The High Velocity Oxygen Fuel (HVOF) process involves lower temperatures and higher velocities than those required by other techniques to obtain high density coatings. It is desirable to know which are the main factors that affect the corrosion behaviour of coated materials. The corrosion behaviour in chloride solution of a 34CrMo4 steel coated with different kinds of powder have been studied. The electrochemical corrosion of the coating-substrate system was characterised by corrosion potential measurements and potentiodynamic polarisations. Microscopic studies have also been performed by means of SEM. The corrosion tests were performed in synthetic marine water (ASTM D-1l41) in the presence of dissolved air. Polarisation resistances have been obtained from potentiodynamic studies. Measurements were carried out on two different (Ti,W)C+Ni coatings, as well as on the coatings obtained from a TiC+Ni-Ti powder which had been previously Ni coated using an electro less method. The best corrosion results were obtained from these last coatings.

The ball-on-disk wear test is widely used in order to determine the sliding wear resistance behaviour of the samples. Although a great number of authors consider the ball-on-disk test to be far from the real situation, the results obtained permit to develop a preliminary idea about the wear behaviour of the tested samples. During the development of the ball-on-disk test a wear track is formed on the test specimen. Scanning white light interferometry can be used to determine the surface damage in the wear track by obtaining images which relate to the surface profile and general morphology. From measurements on interferometric profiles it is possible to determine accurately the track depth and width and hence the volume of the coating removed by wear. It is also possible to determine by optical microscopy the volume of metal lost from the ball. The results obtained have improved the general understanding of the energy absorption during wear and of the observed changes in the coefficient of friction. The results obtained have shown that with the coatings of WC-Co deposited by the HVOF process no volumetric changes occur when using a WC-6%Co sintered ball. Tests on coatings containing TiC or Cr3CZ as the carbide phase or stainless steel coatings show significant losses of material.

The feasibility of using the HVOF process for the thermal spray-forming of free-standing components has been investigated. HVOF spray forming offers a number of potential advantages compared to the established procedure of plasma forming, including increases in component density, and reduction in material decomposition during spraying. Using blends of carbide and superalloy powders in various proportions, HVOF spraying has been successfully used to form free-standing cylinders and cones of various lengths and thicknesses. Microstructural examination of the spray-formed material, using optical microscopy and scanning electron microscopy (SEM), has shown a homogeneous distribution of carbides in the superalloy matrix, with very low levels of porosity. Vickers microhardness has been measured on several sprayed forms. In order to complete the study of the different systems, abrasion (Rubber Wheel Test), friction (Ball on Disk Test) and erosion wear results have been obtained. These wear results have been used in order to evaluate the behaviour of the sprayed samples with a different powder percentage in the blends. Corrosion tests have been done to evaluate the corrosion resistance of the sprayed samples (ASTM D-1411).

The feasibility of using the HVOF process for the thermal spray-forming of free-standing components has been investigated. HVOF spray forming offers a number of potential advantages compared to the established procedure of plasma forming, including increases in component density, and reduction in material decomposition during spraying. Using blends of carbide and superalloy powders in various proportions, HVOF spraying has been successfully used to form free-standing cylinders and cones of various lengths and thicknesses. Microstructural examination of the spray-formed material has shown a homogeneous distribution of carbides in the superalloy matrix, with very low levels of porosity. Refinement of the procedure has allowed reduction of the matrix content, and the forming of fragile materials.